Abstract

We have carried out continuous and step uniaxial extension experiments on monodisperse and bidisperse styrene-butadiene random copolymers (SBR) to demonstrate that their nonlinear rheological behavior can be understood in terms of yielding through breakdown of the chain entanglement network and rubberlike rupture via non-Gaussian chain stretching leading to chain scission, respectively. In continuous extension, the sample with bidisperse molecular weight distribution showed greater resistance, due to double-networking, against the yielding-initiated failure. An introduction of 20% high molecular weight (106 g/mol) SBR to an SBR matrix (2.4 × 105 g/mol) could postpone the onset of nonuniform extension by as much as two Hencky strain units. In step extension, the bidisperse blends were also found to be more resistant to elastic breakup than the monodisperse matrix SBR. Rupture in both monodisperse and bidisperse SBR samples occurred when the finite chain extensibility was reached at sufficiently high rates. It is important to point out here that these results along with the concept of yielding allow us to clarify the concept of strain hardening in extensional rheology of entangled polymers.

Received 12 September 2010Accepted 27 July 2011Published online 02 September 2011

Acknowledgments:

The authors would like to express their sincere gratitude to Dr. Xiaorong Wang from Bridgestone-Americas Center for Research and Technology for providing the SBR samples in this study. This work is supported, in part, by Grant Nos. DMR-0821697 and CMMI-0926522 from the National Science Foundation.